1) Field of the Invention
This invention relates to an optical device for rotating the polarisation of linearly polarised light by 90xc2x0. More specifically it relates to such a device where the beam input to the device and the beam exiting the device are collinear.
2) Discussion of the Prior Art
There are many applications where it is desirable to rotate the plane of polarisation of polarised light through 90xc2x0. These include spectroscopy, where polarisation holds key information such as polarisability, white light interferometry where the reference beam may be required to be rotated by 90xc2x0 in order to see cross polarised components and quantitative metrology applications where measurements are to be made at 0xc2x0 and 90xc2x0.
This has traditionally been achieved by using a half wave plate or by the use of Fresnel rhombs, however there are difficulties associated with these.
A waveplate exploits the birefringence of certain optical materials to impose the required phase shift between the orthogonally polarised components of the light. Individual half wave plates are only effective over a narrow wavelength range, therefore a large number of plates would be required to cover a wide wavelength range. These waveplates must be used individually so that it is not possible to simultaneously cover a wide wavelength range.
Super achromatic waveplates comprise several pairs of matched waveplates made of more than one optical material, so designed that there is chromatic correction over a larger wavelength range. However, achromatic waveplates have a number of drawbacks. The constituent optical elements may be contacted with optical cement, limiting optical power throughput and chromatic correction is still limited for operation at wavelengths below xcx9c300 nm.
Fresnel rhombs achieve wavelength retardation by exploiting the phase difference imposed on s and p polarised light upon total internal reflection. This phase shift is relatively slow varying with wavelength compared to the performance of a waveplate. Nevertheless, the performance is not achromatic, for example a pair of silica Fresnel rhombs designed for half wave operation impose a phase retardation of 180xc2x18xc2x0over the wavelength range 200-400 nm. The deviation from exactly 180xc2x0 of phase retardation results in the light becoming elliptically polarised.
U.S. Pat. No. 4,822,150 describes a device for rotating the polarisation of a light beam by 90xc2x0 while maintaining collinearity comprising a plurality of right angled prisms. The prisms are cemented together such that all interfaces are perpendicular to the path of the light beam. As such, large internal reflection losses at the interfaces will occur. These losses can be reduced by coating the adjoining surfaces of the prisms with an anti-reflection coating, however this leads to an increase in complexity and cost and it is also difficult to design a coating which will operate effectively over a large wavelength range. Furthermore, it is also not possible to provide a broadband, anti-reflection coating which operates in the UV range below xcx9c260 nm. It is possible to reduce reflections using an index matching liquid or cement, but these tend to cause operational difficulties below 260 nm and are not suitable for high power optical throughput. It is suggested that the device can be formed from a single piece of glass, in which case no coating would be required, however the complexity of the device would make this method of manufacture very difficult and costly. This is due to the difficulty in polishing an inner surface on an overhanging structure.
U.S. Pat. No. 5,751,482 describes a prismatic device for rotating the polarisation of a light beam. As with U.S. Pat. No. 4,822,150 the interfaces between the prisms are perpendicular to the path of the light beam so the limitations caused by internal reflections and the problems associated with suitable coatings also apply.
In accordance with the present invention an optical device for rotating the polarisation of a linearly polarised, broadband input beam of light comprises prism means; wherein the input beam is incident normal to a first face and exits normal to a second face of the prism means; wherein the prism means comprises a light transmitting material having a plurality of totally internally reflecting surfaces and one or more interfaces; wherein the totally internally reflecting surfaces are arranged such that the polarisation of the exit beam is rotated by 90xc2x0 relative to the input beam, wherein each interface comprises a pair of parallel surfaces separated by a gap; wherein the parallel surfaces are arranged substantially to satisfy the Brewster condition such that surface reflection losses are minimised; and wherein the input and exit beams are substantially collinear.
The arrangement of the parallel surfaces so as to satisfy the Brewster condition is derived as follows;
Refraction of a light beam passing from a medium with refractive index, ni, through an interface with a medium with refractive index, ni, is governed by Snell""s Law;
ni sin xcex8i=nt sin xcex8txe2x80x83xe2x80x83(1)
where xcex8i and xcex8t are respectively, the angle of incidence of the input beam and the angle of the refracted beam.
The reflection of the incident beam, for example for p-polarised light, is described by the Fresnel reflection coefficient;                               R          "LeftDoubleBracketingBar"                =                                            tan              2                        ⁡                          (                                                θ                  j                                -                                  θ                  t                                            )                                                          tan              2                        ⁡                          (                                                θ                  i                                +                                  θ                  t                                            )                                                          (        2        )            
from equation (2), the reflection coefficient is equal to zero when xcex8i+xcex8t=90xc2x0. Substitution into equation (1) gives the Brewster condition;
tan xcex8i=nt/nixe2x80x83xe2x80x83(3)
At this value of xcex8i, there is no reflection of p-polarised light at the interface.
Arranging the parallel surfaces so as to substantially satisfy the Brewster condition and ensuring that all the internally reflecting surfaces are totally reflecting means that the only significant reflection losses in the device of the present invention are those at the first incident face and the second exit face of the prism means. This leads to a significant reduction in the total losses when compared with prior art devices. The device of the present invention can be manufactured using conventional grinding and polishing techniques. Furthermore, it does not require the use of anti-reflection coatings, enabling operation in the UV range below 260 nm.
The transition of the light beam across an interface introduces a lateral shift into the beam path which is dependent on the wavelength of the light.
Preferably, the interfaces are arranged such that lateral shifting of the beam is mutually cancelled. This ensures that the input beam and output beam are collinear.
Preferably, the prism means comprises three prism sections and two interfaces. This arrangement allows for easy manufacture of the device, however it is clear that alternative arrangements may be constructed. These may include arrangements with odd numbers of interfaces if precise collinearity is not required.
Conveniently, the light transmitting material comprises fused silica. This allows for the construction of a device capable of operation from 200-2000 nm.
Preferably, gap separating the parallel surfaces is air filled.
Preferably, the input beam of light has a wavelength in the range of 200-700 nm.
Preferably, the input beam of light is collimated.
Preferably, the input beam of light is linearly polarised such that the light refracted at internal surfaces is p-polarised.
The Brewster angle is wavelength dependent so zero reflection at a given angle will only be satisfied for a particular wavelength. To allow broadband operation a compromise angle must be chosen for the interfaces which gives minimal reflection over the required spectral range.
Using light transmitting material comprising fused silica, operation between 200-700 nm and an air filled gap preferably, the parallel surfaces are set so that the angle between the direction of propagation of the light beam in the light transmitting material and a normal to the parallel surfaces is in the range 32xc2x0 to 35xc2x0, more preferably the parallel surfaces are set so that the angle is in the range 33.260 to 33.76xc2x0 and ideally the parallel surfaces are set so that the angle is 33.51xc2x0.
For a light transmitting material other than fused silica, such as magnesium fluoride or a different medium filling the gap between the parallel surfaces, such as a liquid, a different angular range would be specified in order to minimise reflection losses.